An operating mechanism of an existing circuit breaker comprises a handle connected with a transmission link to constitute a toggle; a movable contact support device having a plate rotatably mounted on a pivot; and a mechanical interlock that may be disconnected by means of the action of a tripping rod. Such interlock is formed by cooperating a stop catch of the tripping rod with a lock latch rotatably mounted on a mandrel of the plate. A connecting stick is directly connected to the lock latch, and such assembly constitutes a speed-reduction transmission stage which is able to reduce a tripping force; a bimetallic strip is connected to the tripping rod via a unidirectionally driven rotation connecting stick. The existing small-sized circuit breaker mechanism has a smaller releasing force and easily obtains favorable relief pressure, but the movement velocity of a movable contact thereof is low to affect the performances of the circuit breaker.